The present invention relates to an improvement of the aqueous emulsion polymerization for making fluoropolymers. In particular, the present invention relates to the use of particular ethers as chain transfer agents in the aqueous emulsion polymerization for making fluoropolymers with improved properties, in particular fluorothermoplasts and fluoroelastomers with improved properties.
Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc . . . The various applications of fluoropolymers are for example described in xe2x80x9cModern Fluoropolymersxe2x80x9d, edited by John Scheirs, Wiley Science 1997.
The known fluoropolymers include in particular fluoroelastomers and fluorothermoplasts. Such fluoropolymers include copolymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more comonomers such as for example hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P). The term xe2x80x9ccopolymerxe2x80x9d in connection with the present invention should generally be understood to mean a polymer comprising repeating units derived from the recited monomers without excluding the option of other further repeating units being present that derive from other monomers not explicitly recited. Accordingly, for example the term xe2x80x98copolymer of monomers A and Bxe2x80x99 includes binary polymers of A and B as well as polymers that have further monomers other than A and B such as for example terpolymers.
Examples of fluoroelastomers include for example copolymers of TFE and PVE and copolymers of VDF and HFP. The fluoroelastomers may also contain cure site components so that they may be cured if desired. Applications of fluoroelastomers include for example coatings, use as gaskets and seals as well as use as polymer processing aids (PPA). A commercially available processing aid includes for example copolymers of VDF and HFP available from Dyneon LLC under the brand DYNAMAR(trademark) PPA.
Examples of fluorothermoplasts include copolymers of TFE and E (ETFE), copolymers of TFE and HFP (FEP), copolymers of TFE, HFP and VDF (THV) and perfluoroalkoxy copolymers (PFA). Examples of applications of fluorothermoplasts include for example coating applications such as for example for coating outdoor fabric and use as insulating material in wire and cable insulation. In particular ETFE copolymers have desirable properties as insulating material. Further applications of fluorothermoplasts include making of tubes such as for example fuel hoses, extrusion of films and injection molded articles. The extruded fluorothermoplastic articles, in particular films may further be subjected to an e-beam radiation to partially cure the fluorothermoplast.
Several methods are known to produce the fluoropolymers. Such methods include suspension polymerization as disclosed in e.g. U.S. Pat. Nos. 3,855,191, 4,439,385 and EP 649863; aqueous emulsion polymerization as disclosed in e.g. U.S. Pat. Nos. 3,635,926 and 4,262,101; solution polymerization as disclosed in U.S. Pat. Nos. 3,642,742, 4,588,796 and 5,663,255; polymerization using supercritical CO2 as disclosed in JP 46011031 and EP 964009 and polymerization in the gas phase as disclosed in U.S. Pat. Nos. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and especially aqueous emulsion polymerization. Aqueous emulsion polymerization has been generally preferred for the production of fluoropolymers because the process is more environmental friendly than solution polymerization in organic solvents and furthermore allows for easy recovery of the resulting polymer. However, for certain applications, the fluoropolymers produced via the aqueous emulsion polymerization process may have somewhat inferior properties relative to similar polymers produced via solution polymerization. For example, copolymers of E and TFE produced according to the solution polymerization disclosed in U.S. Pat. Nos. 4,123,602 generally have a better heat resistance than similar polymers produced via aqueous emulsion polymerization.
The aqueous emulsion polymerization normally involves the polymerization in the presence of a fluorinated surfactant, which is generally used to ensure the stabilisation of the polymer particles formed. The suspension polymerization generally does not involve the use of surfactant but results in substantially larger polymer particles than in case of the aqueous emulsion polymerization. Thus, the polymer particles in case of suspension polymerization will quickly settle out whereas in case of dispersions obtained in emulsion polymerization generally display good latex stability over a long period of time.
Although the aqueous emulsion polymerization generally involves the use of a fluorinated surfactant, there is also known an aqueous emulsion polymerization process wherein no fluorinated surfactant is added to the polymerization system. Such a polymerization is described in for example U.S. Pat. Nos. 5,453,477 and WO 97/17381. It is taught therein that through careful selection of the initiation system, the polymer particles are self-stabilized through the ionic endgroups of the polymers produced in the process.
To tailor the molecular weight of the resulting fluoropolymer, it has been known to use chain transfer agents. For example, U.S. Pat. Nos. 4,766,190 discloses an aqueous emulsion polymerization to make fluoroelastomers. The chain transfer agents disclosed include C4-C6 hydrocarbons, alcohols, ethers, esters, ketones and halocarbons.
U.S. Pat. No. 4,262,101 also discloses an aqueous emulsion polymerization process. Chain transfer agents used here include halocarbons, aliphatic carboxylic acid esters, ketones, alcohols, malonic esters and lower dialkylglycol.
U.S. Pat. No. 5,608,022 discloses the preparation of a copolymer of TFE and a comonomer of the formula CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2CF(CF3)xe2x80x94O)nxe2x80x94(CF2)mxe2x80x94Z wherein n is 1 or 2, m is 2-4 and Z is CO2R or SO2F with R being C1-C3 alkyl. The polymerization is carried out by dispersing the comonomer in water to a droplet size of less than 2 xcexcm and copolymerizing with TFE in the presence of a C1-C6 alcohol or a C2-C6 ether. The alcohols are being disclosed as the preferred chain transfer agent and the chain transfer agents are being mixed as liquid with the aqueous polymerization medium. It is disclosed that the copolymer can be produced avoiding formation of different kind of polymers in the aqueous medium, in particular the formation of homopolymer of TFE is avoided. In the examples, n-propyl ether is used as a chain transfer agent. However ethers such as n-propyl ether and diethyl ether have been found to cause building of hazardous peroxides which may present a safety issue when used on an industrial scale to manufacture fluoropolymers. The polymers disclosed in this U.S. patent are used as a base material of a salt electrolytic cation-exchanged film.
Alkanes have also been disclosed as chain transfer agents in an aqueous emulsion polymerization. For example, U.S. Pat. No. 3,635,926 discloses the use methane or ethane to make copolymers of tetrafluoroethylene and perfluorovinyl ethers.
WO 00/32655 discloses the use of hydrofluoroethers (HFEs) for the fluoromonomer emulsion-polymerization. These HFEs are taught to be superior to common chain transfer agents such as chloroform in being safer and environmentally benign. These liquid components are also taught to be superior to gaseous chain transfer agents such as ethane because they do not enter the fluoromonomer gas recycle streams. However, the chain transfer activity of HFEs is fairly low, making them generally only practical in producing perfluorinated polymers where only a low chain transfer activity is desired and needed. When using the HFEs in producing partially fluorinated fluoropolymers, one will generally need a large amount of chain transfer, which is undesirable.
In JP 1-129005 there is disclosed the use of dialkyl ether chain transfer agents that have not more than 6 carbon atoms in a suspension polymerization of vinylidene fluoride to produce homo- or copolymers of vinylidene fluoride. Specifically disclosed ethers include dimethyl ether and diethyl ether with the latter being preferred. The polymerization temperature is taught to be between 10 and 25xc2x0 C. It is disclosed that the use of these chain transfer agents allow for control of molecular weight of the vinylidene fluoride polymer without substantially affecting the polymerization rate and the heat resistance of the polymer produced.
A commonly employed chain transfer agent in the production of fluorothermoplasts and fluoroelastomers is diethylmalonate. The use of diethylmalonate is for example recommended in EP 43 948 to produce copolymers of TFE and E, such as for example copolymers of TFE, E, HFP and PVE. However, it has been found that fluoropolymers produced in the presence of this chain transfer agent are susceptible to discoloration, may produce an unpleasant smell and have a high amount of extractable compounds. Also, the fluoropolymers so produced have been found to have a large amount of low molecular weight fraction which causes processing difficulties of the fluoropolymer. Furthermore, the yield of these polymers when produced through aqueous emulsion polymerization in the presence of diethylmalonate would desirably be improved.
Further known chain transfer agents used in aqueous emulsion polymerization include silanes as disclosed in U.S. Pat. Nos. 5,256,745 and 5,208,305. However, also in this instance, it was found that the fluoropolymers produced have undesirable properties such as discoloration and low purity, in particular high amounts of extractable compounds. Additionally, the process disclosed in these patents to produce a bimodal molecular weight distribution of the fluoropolymer is cumbersome, e.g., the polymerization time is long and the polymerization initiation is often retarded.
It would thus be desirable to improve the aqueous emulsion polymerization process so as to produce fluoropolymers with improved properties. It is in particular a desire to produce fluoropolymers that have a high purity, less extractable compounds, less smell, improved processing and less discoloration. It is further desirable to produce partially fluorinated fluoroelastomers and fluorothermoplasts that have improved mechanical and physical properties. Desirably, the chain transfer agents have a high chain transfer activity such that they can be used in low amounts.
The present invention provides a method of making a fluoropolymer comprising repeating units derived from one or more gaseous fluorinated monomers. The method comprises an aqueous emulsion polymerization of the gaseous fluorinated monomer(s) optionally in combination with one or more liquid fluorinated monomers in the presence of an ether selected from the group consisting of dimethyl ether (DME), methyl tertiary butyl ether (MTBE) and mixtures thereof.
It has been found that by using the specific ethers as a chain transfer agent, improved fluoropolymers, in particular fluorothermoplasts and fluoroelastomers can be produced. The fluoropolymers produced with the process of the present invention generally have a high purity and low amounts of extractable compounds including residual chain transfer agent and low molecular weight fractions. The ether chain transfer agents do not act as swelling agent for the polymer, in contrast to e.g. diethylmalonate, and are therefore more easy to remove after polymerization. The fluoropolymers produced generally are more easy to process, produce less smell and are better performing. For example, it has been found that the process of this invention allows for making fluorothermoplastic copolymers of E and TFE that have equal or better physical and mechanical properties such as e.g. heat resistance as fluorothermoplastic copolymers of E and TFE produced by polymerizations in organic solvent. Thus, the fluorothermoplastic copolymers of E and TFE produced with the present invention have excellent properties for use in wire and cable insulation. Further, it has been found that during extrusion of fluoropolymers made according to the invention, less pressure fluctuations occur and the amount of die drool formed is generally reduced as well.
Additionally, the fluoropolymers can be produced in a convenient and fast way and at good yield (e.g. high solids contents), making the process also attractive from an economic perspective. Further because of the higher purity and lower amount of extractable compounds, the polymers produced with the process of this invention will generally also be more environmental friendly. Also, DME and MTBE are ethers that do not generally form peroxide (see Bretherick, L. in: Handbook of reactive chemical hazards; p. 549, Butterworth-Heinemann Ltd 1990, ISBN 0,7506,0706,8) and can therefore more safely be handled than other ethers, in particular other dialkyl ethers, in the polymerization process of this invention.
The ethers are particularly suitable for producing partially fluorinated polymers and because of their high chain transfer activity can be used in low amounts. Additionally, the ethers have high water solubility and as a result thereof, will despite their gaseous nature not readily enter the gas stream of gaseous fluorinated monomers that may be used in an emulsion polymerisation process.
Additionally, it has been found that the ethers can be used to produce fluoropolymers, in particular fluoroelastomers that have a multi-modal, e.g. a bimodal, molecular weight distribution in a single step polymerization. By xe2x80x98single stepxe2x80x99 polymerization is meant that the polymerization can be carried out without having to interrupt the polymerization reaction as has been practiced in the prior art where, in order to produce bimodal fluoropolymers, a first polymer is produced and separately a second one which are then blended together. Further, such multi-modal polymers can be produced with a low level of extractable compounds.
Thus, in a further aspect, the invention also relates to a fluoropolymer having a multi-modal molecular weight distribution, in particular a bimodal molecular weight distribution, and comprising less than 12% by weight, preferably less than 10% by weight, most preferably less than 8% by weight based on the weight of fluoropolymer of extractable compounds as measured by leaving the fluoropolymer in methyl ethyl ketone for 70 hours at 40xc2x0 C.
The process of the present invention also allows for a convenient manufacturing of so-called xe2x80x9ccore-shellxe2x80x9d polymers by feeding different monomers at different stages during the polymerization or by changing the ratio of the monomers during the polymerization process. Using the ether chain transfer agents, an amorphous (elastomeric) core of a desired molecular weight may be polymerized during a first stage of the polymerization and during a second stage later in the polymerization process a semi-crystalline shell with a desired molecular weight may be polymerized. Of course the core can also be semi-crystalline and the shell can be made amorphous (as disclosed in U.S. Pat. No. 6,310,141) or both core and shell may be semi-crystalline (as disclosed in WO 00/69969) or amorphous. The process of the present invention allows for a convenient manufacturing of such core-shell polymers in a one single step polymerization.
Finally, such core-shell polymers can be produced with a low amount of extractable compounds and the invention thus also relates to such core-shell polymers.
According to the present invention, DME or MTBE or a mixture thereof is used as a chain transfer in the aqueous emulsion polymerization.
The amount of chain transfer agent used in the polymerization is generally selected to achieve the desired molecular weight of the fluoropolymer. Typically, the amount of chain transfer agent used will be between 0.1 and 20 g per kg of polymer produced, more preferably between 0.3 and 9 g per kg of polymer produced.
Generally, the aqueous emulsion polymerization process is carried out in the presence of a fluorinated surfactant, typically a non-telogenic fluorinated surfactant. Suitable fluorinated surfactants include any fluorinated surfactant commonly employed in aqueous emulsion polymerization. Particularly preferred fluorinated surfactants are those that correspond to the general formula:
Yxe2x80x94Rfxe2x80x94Zxe2x80x94Mxe2x80x83xe2x80x83(III)
wherein Y represents hydrogen, Cl or F; Rf represents a linear or branched perfluorinated alkylene having 4 to 10 carbon atoms; Z represents COOxe2x88x92 or SO3xe2x88x92 and M represents an alkali metal ion or an ammonium ion. Most preferred fluorinated surfactants for use in this invention are the ammonium salts of perfluorooctanoic acid and perfluorooctane sulphonic acid. Mixtures of fluorinated surfactants can be used.
The aqueous emulsion polymerization process is generally conducted in the commonly known manner. The reactor vessel is typically a pressurizable vessel capable of withstanding the internal pressures during the polymerization reaction. Typically, the reaction vessel will include a mechanical agitator, which will produce thorough mixing of the reactor contents and heat exchange system.
Any quantity of the fluoromonomer(s) may be charged to the reactor vessel. The monomers may be charged batchwise or in a continuous or semicontinuous manner. By semi-continuous is meant that a plurality of batches of the monomer are charged to the vessel during the course of the polymerization. The independent rate at which the monomers are added to the vessel will depend on the consumption rate of the particular monomer with time. Preferably, the rate of addition of monomer will equal the rate of consumption of monomer, i.e. conversion of monomer into polymer.
The reaction vessel is charged with water, the amounts of which are not critical. To the aqueous phase there is generally also added the fluorinated surfactant which is typically used in amount of 0.01% by weight to 1% by weight. The chain transfer agent is typically charged to the reaction vessel prior to the initiation of the polymerization. Further additions of chain transfer agent in a continuous or semi-continuous way during the polymerization may also be carried out. For example, a fluoropolymer having a bimodal molecular weight distribution is conveniently prepared by first polymerizing fluorinated monomer in the presence of an initial amount of chain transfer agent and then adding at a later point in the polymerization further chain transfer agent together with additional monomer.
The polymerization is usually initiated after an initial charge of monomer by adding an initiator or initiator system to the aqueous phase. For example peroxides can be used as free radical initiators. Specific examples of peroxide initiators include, hydrogen peroxide, sodium or barium peroxide, diacylperoxides such as diacetylperoxide, dipropionylperoxide, dibutyrylperoxide, dibenzoylperoxide, benzoylacetylperoxide, diglutaric acid peroxide and dilaurylperoxide, and further water soluble per-acids and water soluble salts thereof such as e.g. ammonium, sodium or potassium salts. Examples of per-acids include peracetic acid. Esters of the peracid can be used as well and examples thereof include tert.-butylperoxyacetate and tert.-butylperoxypivalate. A further class of initiators that can be used are water soluble azo-compounds. Suitable redox systems for use as initiators include for example a combination of peroxodisulphate and hydrogen sulphite or disulphite, a combination of thiosulphate and peroxodisulphate or a combination of peroxodisulphate and hydrazine. Further initiators that can be used are ammonium- alkali- or earth alkali salts of persulfates, permanganic or manganic acid or manganic acids. The amount of initiator employed is typically between 0.03 and 2% by weight, preferably between 0.05 and 1% by weight based on the total weight of the polymerization mixture. The full amount of initiator may be added at the start of the polymerization or the initiator can be added to the polymerization in a continuous way during the polymerization until a conversion of 70 to 80%. One can also add part of the initiator at the start and the remainder in one or separate additional portions during the polymerization. Accelerators such as for example water-soluble salts of iron, copper and silver may preferably also be added.
During the initiation of the polymerization reaction, the sealed reactor vessel and its contents are pre-heated to the reaction temperature. Preferred polymerization temperatures are from 30xc2x0 C. to 80xc2x0 C. and the pressure is typically between 4 and 30 bar, in particular 8 to 20 bar.
The aqueous emulsion polymerization system may further comprise auxiliaries, such as buffers and complex-formers.
The amount of polymer solids that can be obtained at the end of the polymerization is typically between 10% and 45% by weight, preferably between 20% and 40% by weight and the average particle size of the resulting fluoropolymer is typically between 50 nm and 500 nm.
According to a further embodiment of the present invention, the aqueous emulsion polymerization may also be carried out without the addition of a fluorinated surfactant. Aqueous emulsion polymerization that is carried out without the addition of a fluorinated surfactant can be practiced as disclosed in U.S. Pat. No. 5,453,477 and WO 97/17381. According to the emulsifier free aqueous emulsion polymerization disclosed in WO 97/17381 a radical initiator system of a reducing agent and oxidizing agent is used to initiate the polymerization and the initiator system is added in one or more further charges during the polymerization. The ionic end groups formed as a result of the initiator system used in WO 97/17381 are taught to stabilise the fluoropolymer particles in the emulsifier free aqueous emulsion process. Suitable oxidizing agents that can be used include persulfates such as potassium sulfate and ammonium sulfate, peroxides such as hydrogen peroxide, potassium peroxide, ammonium peroxide, tertiary butyl hydroperoxide, cumene peroxide and t-amyl hydroperoxide, manganese triacetate, potassium permanganate, ascorbic acid and mixtures thereof. Suitable reducing agents include sodium sulfites such as sodium bisulfite, sodium sulfite, sodium pyrosulfite, sodium-m-bitsulfite, ammonium sulfite monohydrate and sodium thiosulphate, hydroxylamine, hydrazine, ferrous iron, organic acids such as oxalic acid and citric acid and mixtures thereof.
The amount of oxidizing agent added in the initial charge is typically between 10 and 10000 ppm. The amount of reducing agent in the initial charge is typically also between 10 and 10000 ppm. At least one further charge of oxidizing agent and reducing agent is added to the polymerization system in the course of the polymerization. The further addition(s) may be done batchwise or the further addition may be continuous.
According to a preferred embodiment, an emulsifier free (i.e. without added emulsifier) aqueous polymerization involves an initial charge of an oxidizing agent and a reducing agent and one or more further charges of either the reducing agent or oxidizing agent, but not both, in the course of the polymerization. This embodiment of the invention has the advantage that the aqueous polymerization process can be conducted in an easy and convenient way while still yielding stable polymer dispersions at a high rate and in good yield.
The aqueous emulsion polymerization process of the present invention comprises the polymerization of at least one gaseous fluorinated monomer. According to a particular embodiment of the present invention, the aqueous emulsion polymerization involves a copolymerization of a gaseous fluorinated monomer such as tetrafluoroethylene, chlorotrifluoroethylene and vinylidene fluoride and a comonomer selected from the group consisting of vinylidene fluoride, perfluoroalkyl vinyl monomers, ethylene, propylene, fluorinated allyl ethers, in particular perfluorinated allyl ethers and fluorinated vinyl ethers, in particular perfluorovinyl ethers. Additional fluorinated and non-fluorinated monomers can be included as well. It will be understood by one skilled in the art that when the polymerization involves vinylidene fluoride, the gaseous fluorinated monomer would generally be either tetrafluoroethylene or chlorotrifluoroethylene or a comonomer other than vinylidene fluoride would have to be selected to obtain a copolymer. Examples of perfluorovinyl ethers that can be used in the process of the invention include those that correspond to the formula:
CF2xe2x95x90CFxe2x80x94Oxe2x80x94Rf
wherein Rf represents a perfluorinated aliphatic group that may contain one or more oxygen atoms.
Particularly preferred perfluorinated vinyl ethers correspond to the formula:
CF2xe2x95x90CFO(RafO)n(RbfO)mRcf
wherein Raf and Rbf are different linear or branched perfluoroalkylene groups of 1-6 carbon atoms, in particular 2 to 6 carbon atoms, m and n are independently 0-10 and Rcf is a perfluoroalkyl group of 1-6 carbon atoms. Specific examples of perfluorinated vinyl ethers include perfluoro methyl vinyl ether (PMVE), perfluoro n-propyl vinyl ether (PPVE-1), perfluoro-2-propoxypropylvinyl ether (PPVE-2), perfluoro-3-methoxy-n-propylvinyl ether, perfluoro-2-methoxy-ethylvinyl ether and
CF3xe2x80x94(CF2)2xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94Oxe2x80x94CF(CF3)xe2x80x94CF2xe2x80x94Oxe2x80x94CFxe2x95x90CF2.
Suitable perfluoroalkyl vinyl monomers correspond to the general formula:
CF2xe2x95x90CFxe2x80x94Rdf or CH2xe2x95x90CHxe2x80x94Rdf
wherein Rdf represents a perfluoroalkyl group of 1 to 10, preferably 1 to 5 carbon atoms. A typical example of a perfluoroalkyl vinyl monomer is hexafluoropropylene.
The process of the present invention is preferably used for producing fluoropolymers that have a partially fluorinated backbone, i.e. part of the hydrogen atoms on the backbone are replaced with fluorine. Accordingly, the aqueous polymerization process of the present invention will generally involve at least one monomer that has an ethylenically unsaturated group that is partially fluorinated (e.g. vinylidene fluoride) or not fluorinated (e.g. ethylene or propylene). It has been found that the ethers are highly effective chain transfer agent for use with monomers that have a non-fluorinated or partially fluorinated ethylenically unsaturated group. On the other hand, they can be used to produce perfluoropolymers, i.e. polymers that have a fully fluorinated backbone.
Examples of fluoropolymers that are preferably produced with the process of the invention include a copolymer of vinylidene fluoride and hexafluoropropylene, a copolymer of tetrafluoroethylene and vinylidene fluoride, a copolymer of chlorotrifluoroethylene and vinylidene fluoride, a copolymer of tetrafluoroethylene and ethylene, a copolymer of tetrafluoroethylene and propylene, a copolymer of vinylidene fluoride and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), a terpolymer of tetrafluoroethylene, ethylene or propylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), a copolymer of tetrafluoroethylene, ethylene or propylene and hexafluoropropylene, a copolymer of tetrafluoroethylene, vinylidene fluoride and hexafluoropropylene, a copolymer of vinylidene fluoride, tetrafluoroethylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2), and a co-polymer of a copolymer of tetrafluoroethylene, ethylene or propylene, hexafluoropropylene and perfluorovinyl ether (e.g. PMVE, PPVE-1, PPVE-2 or a combination of PPVE-1 and PPVE-2).
The fluoropolymers that can be produced with the process of the invention are generally amorphous fluoropolymers or semi-crystalline fluoropolymers. A fluoroelastomer is generally a fluoropolymer having elastomeric properties upon curing and will generally not display a melting peak or will have a very minor melting peak, i.e. the fluoroelastomer will generally have little or no crystallinity. Fluorothermoplasts are polymers that generally have a pronounced melting peak and that generally have crystallinity. The fluorothermoplasts according to this invention will generally be melt processible, i.e. they will typically have a melt flow index of at least 0.1 g/10 min. as measured with a load of 5 kg and at a temperature of 265xc2x0 C. as set out in the examples below. Whether the particular fluoropolymer produced is a fluorothermoplast or fluoroelastomer, depends on the nature and amounts of the monomers from which the fluoropolymer is derived as is well known to those skilled in the art.
The fluoropolymers produced with the process of the invention can have a multi-modal including a bimodal molecular weight distribution which can provide advantageous processing properties. In particular, such multi-modal fluoropolymers may be produced by charging no or a small initial amount of ether at the beginning of the polymerization, e.g. prior to the addition of initiator, and one or more further charges of the ether during the polymerization. Accordingly, fluoropolymers with a multi-modal molecular weight distribution can conveniently be produced in a single step polymerization.
Fluorothermoplasts
Fluorothermoplasts that can be produced with the process of the present invention generally will have a melting point between 50xc2x0 C. and 300xc2x0 C., preferably between 60xc2x0 C. and 280xc2x0 C. Particularly desirable fluorothermoplasts that can be produced with the process of this invention include for example copolymers of E and TFE, copolymers of TFE and VDF, copolymers of VDF and HFP, copolymers of CTFE and VDF, copolymers of TFE, E and HFP and copolymers of TFE, HFP and VDF.
Fluorothermoplasts that may be produced in connection with the present invention generally have the advantage of being less susceptible to discoloration, having a decreased amount of extractable compounds and having a high purity and generally are more homogeneous, e.g. the polymer doesn""t show a drift of the melt flow index during polymerizaton. Accordingly, the fluorothermoplasts are generally more easy to process and generally have high temperature resistance, high chemical resistance, same or improved electrical properties good mold release and reduced amount of smell. Further, the fluorothermoplasts when extruded typically produce less die drool.
The fluorothermoplastic polymers that can be obtained with the process of the present invention can be used in any of the applications in which fluorothermoplasts are typically used. For example, the fluorothermoplasts can be used to insulate wires and cables. In particular a copolymer of E and TFE produced with the process of this invention has been found to have highly desirable properties to insulate wires. To produce a cable or wire insulated with a fluorothermoplast according to the invention, in particular a copolymer of E and TFE, the fluorothermoplast can be melt extruded around a central conductor, e.g. copper wire. A conductive metallic layer may be formed around the extruded fluorothermoplast layer to produce for example a heating cable.
The fluorothermoplastic polymers produced may further be used to make hoses, in particular fuel hoses and pipes and can be used in particular in heat exchange applications. The fluorothermoplasts may also be extruded into a film or into so-called mono filaments which may then subsequently be woven into a woven fabric. Still further, the fluorothermoplasts can be used in coating applications for example to coat outdoor fabric or to make injection molded articles.
Fluoroelastomers
In addition to fluorothermoplasts, the process of the present invention also allows for making fluoroelastomers with desirable and improved properties. In particular, the fluoroelastomers produced will generally have a higher purity, a lesser amount of extractable compounds, will generally be less susceptible to discoloration, more easy to process and will typically produce less smell. Additionally, the mechanical and physical properties of the fluoroelastomers may be improved by the process of the invention. For example, a curable fluoroelastomer produced according to the invention may have an improved compression set and may have same or improved permeation properties.
Fluoroelastomers that can be produced in connection with the present invention include elastomers that are not fully fluorinated. The fluoroelastomer may include a cure site component, in particular one or more cure sites derived from a cures site monomer (CSM) to provide a curable fluoroelastomer. Specific examples of elastomeric copolymers include copolymers comprising a combination of monomers as follows: VDF-HFP, VDF-TFE-HFP, VDF-TFE-HFP-CSM, VDF-TFE-PMVE-CSM, TFE-P, E-TFE-PMVE-CSM and TFE-VDF-P-CSM.
To obtain a curable fluoroelastomer, a further cure site component may be included in the polymerization reaction to obtain a curable fluoroelastomer. Generally, the cure site component will be used in small amounts, typically in amounts so as to obtain a fluoroelastomer that has between 0.1 and 5 mol % of cure sites, preferably 0.2 to 3 mol % and most preferred 0.5-2 mol %.
The cure site component may comprise a nitrile group-containing cure site monomer. The cure site component can be partially or fully fluorinated. Preferred useful nitrile group-containing cure site monomers include nitrile-containing fluorinated olefins and nitrile-containing fluorinated vinyl ethers, such as depicted below:
CF2xe2x95x90CFxe2x80x94(CF2)nxe2x80x94Oxe2x80x94Rfxe2x80x94CN
CF2xe2x95x90CFO(CF2)lCN
CF2xe2x95x90CFO[CF2CF(CF3)O]g(CF2O)vCF(CF3)CN
CF2xe2x95x90CF[OCF2CF(CF3)]kO(CF2)uCN
where, in reference to the above formulas: n=1 to 5l; 1=2-12; g=0-4; k=1-2; v=0-6; and u=1-4, Rf is a linear or branched perfluoroalkylene or a bivalent perfluoroether group. Representative examples of such a monomer include perfluoro(8-cyano-5-methyl-3,6-dioxa-1-octene), CF2xe2x95x90CFO(CF2)5CN, and CF2xe2x95x90CFO(CF2)3OCF(CF3)CN.
Alternatively, the cure site component may comprise a fluorinated monomer having a halogen capable of participation in a peroxide cure reaction. Typically the halogen is bromine or iodine. Suitable cure-site components include terminally unsaturated monoolefins of 2 to 4 carbon atoms such as bromodifluoroethylene, bromotrifluoroethylene, iodotrifluoroethylene, and 4-bromo-3,3,4,4-tetrafluorobutene-1. Examples of other suitable cure site components include CF2xe2x95x90CFOCF2CF2Br, CF2xe2x95x90CFOCF2CF2CF2Br, and CF2xe2x95x90CFOCF2CF2CF2OCF2CF2Br. Preferably, all or essentially all of these components are ethylenically unsaturated monomers.
A curable fluoroelastomer composition will generally include the curable fluoroelastomer and one or more curatives such as the peroxide and/or one or more catalysts depending on the type of cure sites contained in the curable fluoroelastomer. Suitable peroxide curatives are those which generate free radicals at curing temperatures. A dialkyl peroxide or a bis(dialkyl peroxide) which decomposes at a temperature above 50xc2x0 C. is especially preferred. In many cases it is preferred to use a di-tertiarybutyl peroxide having a tertiary carbon atom attached to peroxy oxygen. Among the most useful peroxides of this type are 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexyne-3 and 2,5-dimethyl-2,5-di(tertiarybutylperoxy)hexane. Other peroxides can be selected from such compounds as dicumyl peroxide, dibenzoyl peroxide, tertiarybutyl perbenzoate, xcex1,xcex1xe2x80x2-bis(t-butylperoxy-diisopropylbenzene), and di[1,3-dimethyl-3-(t-butylperoxy)-butyl]carbonate. Generally, about 1-3 parts of peroxide per 100 parts of perfluoroelastomer is used.
Another material which is usually blended with the composition as a part of the curative system is a coagent composed of a polyunsaturated compound which is capable of cooperating with the peroxide to provide a useful cure. These coagents can be added in an amount equal to 0.1 and 10 parts per hundred parts perfluoroelastomer, preferably between 2-5 parts per hundred parts fluoroelastomer. Examples of useful coagents include triallyl cyanurate; triallyl isocyanurate; tri(methylallyl isocyanurate; tris(diallylamine)-s-triazine; triallyl phosphite; N,N-diallyl acrylamide; hexaallyl phosphoramide; N,N,Nxe2x80x2,Nxe2x80x2-tetraalkyl tetraphthalamide; N,N,Nxe2x80x2,Nxe2x80x2-tetraallyl malonamide; trivinyl isocyanurate; 2,4,6-trivinyl methyltrisiloxane; and tri(5-norbornene-2-methylene)cyanurate. Particularly useful is triallyl isocyanurate. Other useful coagents include the bis-olefins disclosed in EPA 0 661 304 A1, EPA 0 784 064 A1 and EPA 0 769 521 A1.
When the fluoroelastomer includes a nitrile containing cure site component, a catalyst comprising one or more ammonia-generating compounds may be used to cause curing. xe2x80x9cAmmonia-generating compoundsxe2x80x9d include compounds that are solid or liquid at ambient conditions but that generate ammonia under conditions of cure. Such compounds include, for example, aminophenols as disclosed in U.S. Pat. No. 5,677,389, ammonia salts (U.S. Pat. No. 5,565,512), amidoxines (U.S. Pat. No. 5,668,221), imidates, hexamethylene tetramine (urotropin), dicyan diamid, and metal-containing compounds of the formula:
Aw+(NH3)vYwxe2x88x92
where Aw+ is a metal cation such as Cu2+, Co2+, Co3+, Cu+, and Ni2+; w is equal to the valance of the metal cation; Ywxe2x88x92 is a counterion, typically a halide, sulfate, nitrate, acetate or the like; and v is an integer from 1 to about 7. Still further ammonia generating compounds are disclosed in PCT 00/09603.
Fluoroelastomers, in particular VDF containing fluoroelastomers, may further be cured using a polyhydroxy curing system. In such instance, it will not be required that the fluoroelastomer includes cure site components. The polyhydroxy curing system generally comprises one or more polyhydroxy compounds and one or more organo-onium accelerators. The organo-onium compounds useful in the present invention typically contain at least one heteroatom, i.e., a non-carbon atom such as N, P, S, O, bonded to organic or inorganic moieties. One useful class of quaternary organo-onium compounds broadly comprises relatively positive and relatively negative ions wherein a phosphorus, arsenic, antimony or nitrogen generally comprises the central atom of the positive ion, and the negative ion may be an organic or inorganic anion (e.g., halide, sulfate, acetate, phosphate, phosphonate, hydroxide, alkoxide, phenoxide, bisphenoxide, etc.).
Many of the organo-onium compounds useful in this invention are described and known in the art. See, for example, U.S. Pat. No. 4,233,421 (Worm), U.S. Pat. No 4,912,171 (Grootaert et al.), U.S. Pat. No 5,086,123 (Guenthner et al.), and U.S. Pat. No 5,262,490 (Kolb et al.), U.S. Pat. No. 5,929,169, all of whose descriptions are herein incorporated by reference. Another class of useful organo-onium compounds include those having one or more pendent fluorinated alkyl groups. Generally, the most useful fluorinated onium compounds are disclosed by Coggio et al. in U.S. Pat. No. 5,591,804.
The polyhydroxy compound may be used in its free or non-salt form or as the anionic portion of a chosen organo-onium accelerator. The crosslinking agent may be any of those polyhydroxy compounds known in the art to function as a crosslinking agent or co-curative for fluoroelastomers, such as those polyhydroxy compounds disclosed in U.S. Pat. No. 3,876,654 (Pattison), and U.S. Pat. No 4,233,421 (Worm). One of the most useful polyhydroxy compounds includes aromatic polyphenols such as 4,4xe2x80x2-hexafluoroisopropylidenyl bisphenol, known more commonly as bisphenol AF. The compounds 4,4xe2x80x2-dihydroxydiphenyl sulfone (also known as Bisphenol S) and 4,4xe2x80x2-isopropylidenyl bisphenol (also known as bisphenol A) are also widely used in practice.
Prior to curing, an acid acceptor is mixed into a fluoroelastomer composition that comprises a polyhydroxy cure system. Acid acceptors can be inorganic or blends of inorganic and organic. Examples of inorganic acceptors include magnesium oxide, lead oxide, calcium oxide, calcium hydroxide, dibasic lead phosphite, zinc oxide, barium carbonate, strontium hydroxide, calcium carbonate, etc. Organic acceptors include epoxies, sodium stearate, and magnesium oxalate. The preferred acid acceptors are magnesium oxide and calcium hydroxide. The acid acceptors can be used singly or in combination, and preferably are used in amounts ranging from about 2 to 25 parts per 100 parts by weight of the fluoroelastomer.
A curable fluoroelastomer composition may comprise further additives, such as carbon black, stabilizers, plasticizers, lubricants, fillers, and processing aids typically utilized in fluoroelastomer compounding can be incorporated into the compositions, provided they have adequate stability for the intended service conditions.
Carbon black fillers are typically also employed in elastomers as a means to balance modulus, tensile strength, elongation, hardness, abrasion resistance, conductivity, and processability of the compositions. Suitable examples include MT blacks (medium thermal black) designated N-991, N-990, N-908, and N-907, and large particle size furnace blacks. When used, 1-70 phr of large size particle black is generally sufficient.
Fluoropolymer fillers may also be present in the curable compositions. Generally, from 1 to 50 parts per hundred fluoroelastomer of a fluoropolymer filler is used. The fluoropolymer filler can be finely divided and easily dispersed as a solid at the highest temperature utilized in fabrication and curing of the fluoroelastomer composition. By solid, it is meant that the filler material, if partially crystalline, will have a crystalline melting temperature above the processing temperature(s) of the fluoroelastomer(s). The most efficient way to incorporate fluoropolymer filler is by blending latices; this procedure including various kinds of fluoro polymer filler is described in U.S. application Ser. No. 09/495600, filed Feb. 1, 2000.
The curable compositions can be prepared by mixing the fluoroelastomer, the curatives and/or catalysts, the selected additive or additives, and the other adjuvants, if any, in conventional rubber processing equipment. The desired amounts of compounding ingredients and other conventional adjuvants or ingredients can be added to the unvulcanized fluorocarbon gum stock and intimately admixed or compounded therewith by employing any of the usual rubber mixing devices such as internal mixers, (e.g., Banbury mixers), roll mills, or any other convenient mixing device. For best results, the temperature of the mixture during the mixing process typically should not rise above about 120xc2x0 C. During mixing, it is preferable to distribute the components and adjuvants uniformly throughout the gum for effective cure. The mixture is then processed and shaped, for example, by extrusion (for example, in the shape of a hose or hose lining) or molding (for example, in the form of an O-ring seal). The shaped article can then be heated to cure the gum composition and form a cured elastomer article.
Pressing of the compounded mixture (i.e., press cure) usually is conducted at a temperature between about 95xc2x0 C. and about 230xc2x0 C., preferably between about 150xc2x0 C. and about 205xc2x0 C., for a period of from 1 minute to 15 hours, typically from 5 minutes to 30 minutes. A pressure of between about 700 kPa and about 20,600 kPa is usually imposed on the compounded mixture in the mold. The molds first may be coated with a release agent and prebaked. The molded vulcanizate is then usually post-cured (e.g., oven-cured) at a temperature usually between about 150xc2x0 C. and about 300xc2x0 C., typically at about 232xc2x0 C., for a period of from about 2 hours to 50 hours or more depending on the cross-sectional thickness of the article. For thick sections, the temperature during the post cure is usually raised gradually from the lower limit of the range to the desired maximum temperature. The maximum temperature used is preferably about 300xc2x0 C., and is held at this value for about 4 hours or more.
The curable fluoroelastomer compositions are useful in production of articles such as gaskets, tubing, and seals. Such articles are produced by molding a compounded formulation of the curable composition with various additives under pressure, curing the part, and then subjecting it to a post cure cycle. The curable compositions formulated without inorganic acid acceptors are particularly well suited for applications such as seals and gaskets for manufacturing semiconductor devices, and in seals for high temperature automotive uses.
The invention will now be further illustrated with reference to the following examples without the intention to limit the invention thereto. All parts and percentages are by weight unless indicated otherwise.